Effect of sintering time on the microstructure, flexural strength and translucency of lithium disilicate glass-ceramics

Glass-ceramics derived from the SiO2–Li2O–K2O–Al2O3–ZrO2–P2O5 system were produced by a two-stage sintering cycle with a primary thermal treatment at 700 °C followed by a secondary thermal treatment at 900 °C. The periods during which the materials were subjected to each temperature level (700 °C/900 °C) were the following: A, 1 h/1 h; B, 2 h/2 h; C, 3 h/3 h; D, 4 h/4 h; E, 5 h/5 h; F, 6 h/6 h; and G, 8 h/8 h. The X-ray diffraction results revealed that all groups had a similar crystalline structure though the sintering times were different. The main crystalline phase corresponds to Li2Si2O5 and the minor phases correspond to Li2SiO3 and Li3PO4. Scanning electron microscopy analysis showed an interlocking microstructure of rod-shaped Li2Si2O5 crystals in all specimens. However, the glass-ceramics produced with different sintering times showed different crystal sizes and spatial distribution. The glass-ceramic sintered during 5 h in each temperature level (700 °C/900 °C) showed the highest flexural strength (308 ± 25 MPa). The translucency parameters (TP) of the glass-ceramics studied were not affected by sintering time since similar TP values were observed in all specimens.